The copper should not corrode in the time you need to run a longer wire. You could also have put a wire nut on it.
If this wire is going to a surround you could probably splice in an additional length to make your run; purists may scoff but for a surround this is not as critical. You could either use wire nuts or solder the additional wire length to reach the speaker in question.
Also, regarding your sub, does it have its own crossovers or will you be relying on your receiver's LFE channel to do the bass management. If the previous sentence does not make sense to you, then read on below. If it does make sense, then if your receiver does bass management, you need to connect your sub so that you bypass its internal crossover.
A sub reproduces low frequencies below the frequency response of regular speakers. Setting crossovers means you have to decide at what frequency (and below) you will rely on the sub to play sounds below the low frequency limit of your speakers. This frequency setting is either done by the receiver itself or by the listener (if the receiver does not do automatic calibration).
Most modern receivers use a test microphone for system calibration; after everything is connected, the receiver will send out test frequencies and set up the crossover frequency (where the low frequencies "cross over" from the speakers to the sub). Then the receiver manages the bass output (bass management) so you do not have to use the crossover settings for your sub, assuming it has that capability.
If you have to do it manually, search online: here are lots of good articles about subwoofer setup out there.
You should buy a system calibration/test DVD to help you tweak your system so you get the most out of it after you run your connections.
Good luck!